1. Field of the Invention
This invention relates to semiconductive shields for use in electrical conductors such as power cables that exhibit improved physical properties and processability compared to known semiconductive shields or insulation.
2. Description of the Related Art
Power cables typically include a core electrical conductor, an overlaying semiconductive shield, an insulation layer formed over the semiconductive shield, and an outermost insulation shield. The semiconductive shield used to screen the electrical conductor is conventionally formed by dispersing various furnace-type carbon blacks in an ethylene copolymer resin base. Commercially available high performance semiconductive shield compositions typically have a high viscosity due to the high carbon black loadings needed to achieve adequate conductivities. However, a high carbon black loading with high viscosity results in poor processing.
The surface smoothness of an extruded article can be improved by using carbon blacks with larger diameter particles or lower surface area. However, resistivity of carbon black based material is related to particle size. The larger carbon black particles result in higher, or poorer, resistivity. Hence, as particle size is increased in order to improve the surface smoothness, the resistivity of the material is increased to an undesirable level.
Acetylene black belongs to an intermediate class between graphite and amorphous carbon, and it has a large specific surface area and a stereostructure wherein primary particles are chained to one another. Acetylene black is carbon black of high purity, e.g., typically less than 1% based on weight of the composition of inorganic impurities, more typically less than 0.1% of inorganic impurities.
Acetylene black has been used in semiconductive shield applications but high loadings, e.g., above 37 weight percent, of it leads to the formation of acids in the extruder which can corrode and abrade the extrusion die tooling, resulting in equipment degradation and cable dimension variations over time.
Thus, there exists a need to improve the processing and conductivity at reduced carbon black levels of semiconductive shield materials.